Method of preparing dicyandiamide



Feb. 25, 1947. J. L. osBoRNE 2,416,545'

' METHOD 0F PREPARlNG DICYANDIAMIDE l Filed Dec. 19, 1944 I INVE-NTORl/of//y @550w/Vf,

ATTORN EY Patented Feb. 25, V1947 2,416,545 METHOD OF PREPARINGDICYANDIAMIDE John L. Osborne, Elizabeth, N.'J., assignor to AmericanCyanamid Company,

New York,

- v N. Y., a corporation of Maine Application December 19, 1944, SerialNo. 568,895

This invention relates toa process of preparing dicyandiamide from crudecalcium cyanamide. The principal object-'ofthe present invention is toprepare dicyandiafmide by polymerizing cyanamide obtained-from crudecalcium cyanamide in an efficient manner'and in good yields and purity,`While suppressing the formation of undesirable by-products. n

An important adjunct of the 'invention 'resides in facilitating contactbetweena gaseous precipitant such as CO2 with calcium cyanamide such asby use of what may be termed a CO2 carrier, that is, by the use of amaterial which will react with CO2 and the resulting czirboriateorVbicarbonate go into solution-Where it will then readily react WithCaCNz. Such a situation makes possible a better Contact of the gaseousprecipitation agent with the calcium cyanamide than if such agent wereused strictly in gaseous form, and consequently can result in a' savingof up to 25% of the CO2. f

In patentl application Serial No. 568,;392 it is proposed to carry out aprocess substantially identical With that of the present applicationemploying the alkali carbonatos as the CO2 carrier. In Serial No.568,893 methyland ethyla-mines are employed, and in Serial No. 568,894guanylurea and guanylurea carbonate are employed.

Heretofore, as disclosed in the prior art, carbon dioxide has been usedto precipitate calcium 'carbonate from aqueous slurries of crude calciumcyanamide. However, such processes involve, essentially, gaseous-liquidreactions. When pure carbon dioxide is used, such reactions necessitatebulky, expensive and unusually complex mixing and diiusion apparatus inorder to comminute the carbon dioxide bubbles and effect a bettercontact of vthe carbon "dioxide and liquid solvent, When stack gases areused as a source of carbon dioxide in these prior processes, thedisadvantages become still greater, for then the use of gaseous-liquidreactions in the preparation of cyanamide and dicyandiamidenecessitates, the processing of large volumes of gases. This requiresunusually large, bulky equipment' and necessitates treatment overextended time Yintervals'. The operation of such processes as disclosedin the prior art is accordinglyquite ineiicient, principally because ofthe low solubility of carbon dioxide in water. Furthermore, these priorprocesses also fail to overcome the disadvantages at;-l tendantupon thelcvvsolubility of lime and calcium cyanamide inthe Water'used to make upthe aqueous slurry. u

A The present process involves a'novel method of facilitating thereaction betweenv carbon'dioxide and crude calcium cyanamide slurries.It relates to a simple process-for bringing about the better solutionofr carbon dioxide in the water'forming the' slurry. Thus, it has been-found that bythe llo 4 claims. (ci. 23-190) addition of Va suitablewater soluble compound capable of carrying carbon dioxide in combinationthere is effected what is essentially a marked increase in thesolubility'of carbon dioxide. Fur; thermore the reaction of CO2 andcalcium cyanamide is completed more eiliciently. This is due to the factthat the process is effected as a liquid to liquid rather than as a gasto liquid type of reaction. The solubilizing of the lcarbon dioxideconverts the latter into a more readily available condition, diffusedthroughout the liquid, and hence effects the reaction as one closelyapproximating an ideal liquid to liquid type of contact.v

These novel features in the process ofthe present invention makepossible the use `of smaller", less complicated apparatus. The presentprocess also shortens'the time necessary to complete the reaction ofcarbon dioxide With the lime and the CaCNz. At the same time it makespossible the use of lower pressures of carbon dioxide while effecting agreater concentration of carbon dioxide in solution. This results in amore efcient process and a more uniform quality of product. Among thecompounds capableV of reacting in this mannerare the following: thealkali metal carbonates, such as sodium and potassium carbonate,guanidine and guanylurea both as such and as their carbonates, theethanolamines such as the mono, di and tri forms, and also the alkylamines such as the mono and poly methyl and ethyl amines. u

More specifically, in accordance with `one embodiment of this invention,it is proposed that an ethanolamine such as the mono, di or triethanolamine be dissolved in the liquor used to extract cyanamide fromthe crude Vcalcium cyanamide slurry 'and'that'carbondioxide be passedinto the resultant slurry." It Will be found that the amine 'serves todissolve CO2 Whichreacts with calcium cyanamide and accelerates theformation of calcium' carbonate. The amine reacts with the CO2 to formthe corresponding carbonate which in turn reacts with the calcium of thecrude calcium cyanamide. It also reacts with the lime which is presentin the slurry'as calcium hydroxide. This reforms the amine and aprecipitate of calcium carbonate. The amine absorbs additional carbondioxide at a very rapid rate, reforming the corresponding ethanolaminecarbonate which then reacts with the calcium acid cyan'amide in the exetraction liquor. This yields calcium carbonate as a precipitate and freecyanamide. This solution may be then concentrated as by evaporation, andany cyanamide therein not already polymerized, polymerizes to formdicyandiamide which is readily extracted by ltration.

It is to be noted that the use of the process of this invention not onlymakes possible a greater concentration of carbon dioxide in Vthe slurrybut also results in thel` presence of carbon-dioxide in a more readilyreacting form than the usual gaseous state. This is due tothe ladditionto the slurry of the above mentioned compounds. All of these compounds-have a high ainity `for 3 carbon dioxide which is held in combination 1therewith. Hence these are capable of serving as carriers of carbondioxide held in combination and readily available for further reactionas'with i the calcium of the slurry. 'Iihesecompounds make possible amore complete absorption and solution of carbon dioxide in the motherliquor than i couldbe effected if the .carbon dioxide were bubbledthrough a slurry containing no such dioxide under several-atmospherespressure. Becausev of the lo-w pressurc-solubilizing stepl vchar--unnecessary lto carry out the process :in suchexpen-sive air-tightautoclavesf and pressure vessels as heretofore used.

The yaccompanying flow@ sheet diagrammatically illustrates a preferredembodiment of the Furthermore with these, L

` acteristic of the process of this invention, itis invention. As shown,a.r quantity of an aqueous solution of the carbon' dioxide carrier andcrude calcium cyanamide inl proper vproportions and i concentrations isworked up into a slurry with water,vor with a recycled portion of themother or wash liquorfrom a previous batch. rlhe mixing tank is heatedor cooled in order to maintain the temperature within 'optimum limits`so as tov obtain desirable yields and avoid excessive-dei compositiondue to side'- reactions which result in the production of NH3, urea andthe like. The slurry may be prepared in an atmosphere of car- .bondioxide under superatmospheric pressure in i .an autoclave or,alternatively, carbon dioxide is bubbl'ed -through the slurry atatmospheric presi sure. l an additional time interval in a second tankto The reactants are mixed, preferably, for

increase the extraction of the calcium cyanamide, after which the slurryis ltered. In order to increase the concentration of nitrogenV presentin Ythe v,form of cyanamide .and later as dicy.andi amide,V thenr-iotherl liquor forming kthe clear nl ltrate is recycled a'number oftimes and reslurried with additional portion-s of crude .calcium.oyanamiclethe combined recycled mixture .being then iinally ,filteredWhen the mother liquor at tains' its optimum concentration of `cyanamideor dicyandiamide it is withdrawn throught-he by-n pass for furthertreatment. i mined number of recycling steps including the re-`slurrying of the filter cake, the filter cake re- `maining `as a sludgein the final ltration step is washed with a small amount of wash liquor`in forder to remove the greater portion of the entrainedY vcarbondioxide carrier left therein. This :wash :liquor is recycled toform analiquot part of a new batch or slurry of calcium cyanamide. lZthe sludgeremaini-ngin `the `filter' is re- I, :moved from the system by scrapingthe filter `plates clean 'of the spent sludge. A new slurry After apredeteris then prepared as above described, theV cycling and' recyclingsteps are carried out again termlnating in a nal ltration and `removalvfrom `the extraction cycle of the mother liquor coni Vtain-ing itsoptimum yconcentration:of cyanamide or-dicyandiamide. This optimumconcentration may be oneA either low in -cy-anamide and high,-v

in dicyandiamide-or lthe reverse. ,The illlllltl 4 situation is a highlydesirable one inasmuch as by building up the content of dicyandiamide inthe mother liquor, that is, by' polymerization of the cyanamide todicyandiamide, as .the former is freed from calcium cyanamide, theliquor goping through the by-pass will contain more and moredicyandiamide making evaporation at this stage unnecessary. Thatdicyandiamide can be built up in the mother liquor `will be apparentwhen it is considered that the liquor at this stagei is .ofsufcientlyhigh alkalinity and temperature to facilitate polymerizationof the cyanamide to dicyandiamide. This situation will be more fullydiscussed hereinafter.

In addition to the above batch process, the process can bereadilyAeffected .by .continuouslir drawing .ofi a fractional lpartof themother liquor or concentrate through the .by-pass for further specialtreatment. Thefremaining portion is'returned. or recycled to .theuflrsttank` where more crude .calcium Vcyanamidel and .carbon dioxide as wellaswash liquor and ymother liquorfrom other sources, as shown, can beadded to maintain the recycling volume constant.

The motor liquor withdrawn through the bye pass may be concentrated inan evaporator, pref-f erably by lowtemperature Vacuum distillation, toremove the excess water. vThe carbon dioxide held in combination bythecarrier issimultanef ously evolved With the result that the alkalinityof the solution maybe increased to that of a solution having a pH of11.0 or thereabouts, where; upon practically all of the cyanamidepolymerizes to dicyandiamide. The dicyandiamide, contain-,- ing perhapsa smallamount of cyanamide in the entrained liquor, is readily separatedout by cooling and filtering the solution. The motor liquor Yiormingtheltrate is recycled as shown to form a new slurry. If contaminated undulywith decomposition products, such as urea for example, it may bepreferable to discard this ltrate periodically.

When vacuum evaporation of the by-passed mother liquor is conducted at alow temperature, i. e., about 30 C. to 35 C.,.most of the cyanamidepresent polymerizes to form dicyandiamide due to the high alkalinity ofthe solution. Any re maining free cyanamide subsequently polymerizes onstorage. However, if complete conversion of the ,cyanamide todicyandiamide is desiredV iin-'- mediately, the evaporation step may beomitted, particularlyii" the solution is stronger than about l0 or 12%.Thus, if `theso'lution has a concentration of 15% or higher, it is notnecessary to evaporate it further. Instead polymerization can be carriedout by a carefully Vcontrolled heating step here which elects aconversion of the cyanamide to dicyandiamide. The dicyandiamideseparates out in crystalline form and is easily collected by cooling theliquor and filtering as above described.

In connection with the building up of dicyandi. amidein the motherliquor at theexpense lof the cyanamide, and prior to by-passing themother liquor .to the recoverysystem as above set forth, the .followingis-an` important consideration. It would, ofcourse, be desirable tooperate the cycle under such. conditions that most, if not all, of thecyanamide went directlyto dicyandiamide reasonably as fastas -thecyanamidewas freed, `maintaining enough water in the system to keep .thedicyandiamide in solution so that When the final liquor V,reached theevaporatorthe vdicyandi- -amide could be recovered therefrom either by:evaporation orchillinggor both.

" The above may be readily accomplished by maintaining the pH of thesystem at 7.0 or above. While high temperatures are desirable in orderto increase the rate of polymerization of cyanainide to dicyandiamide,yet the temperature is not as important as the pH.

It will be apparent that as the calcium cyanamide is fed to the mixingtank the pH has a tendency to rise, whereas when the CO2 is fed to themixing tank the pH has a tendency to fall. Thus, it is asimple matter,by judicious control of these two reactants, to maintain the pH between7.0 and 12.0 so as to maintain desirable cyanamide polymerizingconditions. This can be very simply accomplished by keeping the calciumcyanamide feed ahead of the carbon dioxide feed. The temperature of theextraction cycle may be maintained at from 30 C. to boiling, the exactiigure depending upon the existing pH. For a given pH within the desiredrange, the higher the temperature, the faster the polymerization. At thesame time, if operation conditions require a stated temperature, the lpHmay be adjusted accordingly.

For instance, Where the temperature of the reaction mass is 50 C.cyanamide may be readily converted to dicyandiamide at a pI-I of 9.6While l.at higher temperature such as 80 C. the optimum 'pHisabout 9. Atboiling temperatures the pH 4"may ldrop as low as 8 while stillobtaining polymerization of cyanamide to dicyandiamide at good rates.

The increase in ytemperature to increase the rate of polymerization vcanbe very easily accomplished where flue gases are used as a source ofcarbon dioxide since such gas as discharged from a boiler plant willgenerally be at a temperature bf204 C. or higher. Thus the sensible heatof such gases may be used to raise the temperature of the reaction massand to supplement if neces- 'sary the heat evolved there. In the eventthat the temperature in the mixing tank tends to rise to undue heights,this can be readily controlled by proper cooling of the stack gases.Inasmuch as such gases contain carbon dioxide generally to the extent ofabout 12%, the other constituents being in the nature of diluents, alarge volume of gas may lbe passed through the magma in the mixing tankso as to accurately control the temperature desired there. A 1fGenerally speaking, optimum quantities of cyanamide may be released froma calcium cyanamide slurry in the presence of CO2 where the pH ismaintained no lower than '7.0 and preferably from 8.0 to 11 or slightlyhigher with a temperature of from 30 C. to boiling. A rise intemperature for any given pH will, of course, increase Thus it isentirely possible to maintain they ic ycleunder such conditions of pHand temperature that there will be little. or no cyanamide goingtl'irough the by-pass but on the contrary this by-passed mother liquorWill'be almost exclusiyelyia desirably high concentration of aquedus,dicyandiamide. VSolutiongcontaining' a vminimum quantity of vundesirabledecomposition prod purity which may be filtered from the mother` liquorand recovered as such. The mother liquor from the dicyandiamide recoverysystem may then be returned to the cycle as indicated.

- Only a small amount of the carrier is lost from the system by failureto wash thefirst filter cake. Another small portion of the carrier maybe lost by failure to wash the final filtercake. However, by fiushingthese filter cakes with wash water the carrier therein is easilyremoved. Thus, the entrained carrier may be removed from the finalproduct by washing it out of the lter cake with a small portion of waterand simultaneously leaving the relatively insoluble dicyandiamide on thefilter. Hencev it is unnecessary'to renew the entire supply of thecarrier for each extraction. Instead, the addition of a small portion ofthe carrier or an aqueous solution thereof to the recycled mother liquorat intervals during the process or after each extraction cycle is ampleto increase the concentration of the carrier in the mother liquorsuciently for effective re-use upon further addition ofV carbon dioxide.

Due to the inclusion of a filter in the extraction and/or conversioncycle, the solids are being continuously removed. Thus any desirableconcentration of solubles up to and just short of saturation in theliquor for any desirable work ing temperature may be had without makingthe recycle magma unhandlable due to solids.

The reactions involved may be briefly represented by the followingequations when sodium carbonate is used as the carbon dioxide carrier:

The sodium bicarbonate formed serves as a highly accessible source ofcarbon dioxide and reacts with the crude calcium cyanamide toprecipitate calcium carbonate as follows:

amine such asmono, di or tri ethanolamine,

mono, di or tri -methyl or ethyl amine, and the like, are used as thecarbon dioxide-carrier, it being understood that when the free organicbase is used, the first CO2 entering the system converts into thecarbonate.

The following examples are given toA more clearly illustrate the processinvolved.

Eample 1 i000 parts of water, 250 parts of crude calcium cyanamide andZOparts of mono ethanolamine are Worked up into a slurry and sufficientcarbon'.

, dioxide, -say parts, added thereto to precipi-l tate the calcium andto react withthe amine and give the corresponding .bicarbonate solutionhav,-v ing a pH of about 8.4. The resulting slurry is mixed further inanother mixing kettle and then filtered. The filtrate is evaporated at atemperature'below 35 rC., cooled and the crystalline precipitate,predominantly dicyandiamide contaminatedl with such small amounts ofcyanamide as are present in the entrained liquor, recovere byfiltration. .i

Y rier. p

100 parts of'crude calcium cyanamide are gradually fed into a 300 partmixture of wash water containing .some carrier and mother liquorcontaining the amine carbonate 'from a previous batch extraction. Theslurryf'is fortied with enough mono ethanolamine to brirng itsequivalent up to 20 parts andy 50 parts of carbon dioxide aresimultaneously supplied while maintaining the pH at 8.4 and keeping thetemperature below 35 C. The resulting slurry is passed to another mixingkettle and then filtered. About Y 275 parts of iiltrate are `obtainedcontaining from to 12% of cyanamide. The -sludge on the lter is washedand the wash water recycled to form part of a new batch. Due'to the lowconcentration of values in the filtrate, the latter is furtherevaporated under vacuum and below 35,"r C. until crystals begin to formwhereupon the liquid is chilled Aand-filtered. The yield ispredominantly dicyandiamide, the cyanamide which rst formed havingpolymerized to dicyandiamide, because ci the increased pH. The nitrateis recycled to form the mother liquor-for the next batch.

Example 4 A batchr similar to that described in Example 3 is subjectedto the vsame series of steps to obtain a corresponding 275 part ltrate.In order to obtain .a .practically complete conversion todicyandiarnide, the solution ofcyanamide comprising the 275 part ltrateis evaporated at a temperature of about 70 C., although othertemperatures can also be lused up to boiling. A highly alkaline solutionis formed by the evolution of CO2 'from the solution containing thecarrier. This results in a solution having a pH between 8.4 and 11.4.,when the higher temperatures above given are used to accelerate thepolymerization of the cyanamide to dicyandiamide. Since the latteris'relatively insoluble in the cooled mother liquor, the dicyandiamideisy readily separated by ltering the crystalline precipitate. The ltrateis then recycled to form another slurry of calcium cyanamideV forsubsequent extraction,

The extraction step of the process described in the preceding examplesis preferably carried out at a pH of approximately 8.4. Various changesmay, however, be madeV in the particular steps, thus for example, ifstack gases are used as the source of carbon dioxide it Ymay bepreferable to strip the CO2 from the Vstack gases with an aqueoussolution of a Vcarrier and thereafter adrnix the carrier solution withthe slurry, thereby precipitating CaCOs, the whole mixture being leddirectly into the first mixingtank. Y

lA feature of this invention is the relatively complete control of theprocess andthe relatively rapid initiation of the reaction obtained bytheY use of an ethanolamine or a similar CO2 car` in solution,maintaining a pH between 7.0 and 8 When the `above examples are repeatedwithout the use of a CO2 carrier substantially lon-ger periods oftimeare required to effect the calcium precipitation. absorption of CO2without the carrier, proportionately more CO2 is required because vmoreCO2 passes through the slurry unabsorbed.

In the examples given, NazCOa, KzCOs, guanidine Vor guanylurea, eitheras such or as their carbonates, or a di or tri ethanolamine or .mono di,or tri, methyl or mono, di or tri ethyl amine can be used in place ofthe mono ethanolamirre. In case the other ethanolamines mentioned .areused the following number of parts thereof should be present:

\ Parts Diethanolamne i 35 Triethanolarnine -..1.50

In each instance dilute solutions of the respective carbon dioxidecarriers of from 3% to 5% or even 10% strength fall in a desirablerange.

What is claimed:

l. A method of preparing dicyandiamide which includes slurrying calciumcyanamide in the presence of CO2 and a substance chosen from the groupconsisting of mono, di-, and tri-ethanolamine, in suilicient water tomaintain all the dicyandiamirie formed in Solution, maintaining a .pHbetween 7.0 and 12.0 and a temperature between 30 C. and boiling,whereby calcium cyanamide is converted to calcium carbonate anddicyandiamide, separating the calcium carbonate from the mother liquor,and recovering the dicyandiamide from the latter.

2. A method oi preparing dicyandiamide which includes slurrying calciumcyanamide in the presence of CO2 and a substance chosen from the groupconsisting of mono, diand tri-ethanolamine, in sulcient water tomaintain all the dicyandiamide formed in solution, maintaining a pHbetween 7,0 and 12.0 and a temperature between 30 C.. and boiling,separating out the calcium solids, recirculat'ing the mother liquortherefrom withfresh additions of calcium cyanarnide 'and CO2, lteringout the solids with each recirculation, until the mother liquor from thesolids removal is just short'of saturation with dicyandiamide, andremoving the latter therefrom.

3. A method of preparing dicyandiamide which includes slurrying calciumcyanamide in the presence of CO2 and mcnoethanolamine, insufficientwater to maintain all the dicyandiamide formed short of saturation withdicyandiamide, and re-l i moving the latter therefrom.

' 4. The method of claim 3 with the additional step of recirculating thedicyandiamide mother liquor tothe cycle.

JOHN L. OSBORNE.

REFERENCES rCITED The following references are of record inthe le ofthis patent:.

UNITED STATES PirIErI'rs. y

Number `Name f Date s 2,337,483 Osborne .f Dec. 2.1, v1943 Re.. 18,953.BOSOIIIS n 'Sept'. 26, 51933 Also due to the lesser speed of"

